Starting Methods of Induction Machine

7/31/2019 Starting Methods of Induction Machine

1/18

STARTING METHODS OF

INDUCTION MACHINE


2/18

Why we need starters?

As it is seen that a 3 phase induction motor haspositive finite starting torque T when slip s=1. thismean that 3-pahse induction motor is a self-startingmotor and begins to rotate on its own when

connected to a 3-phase supply. At the instant of starting 3-phase induction motor

behaves like a transformer with a short-circuitedsecondary.

Consequently, a 3-pahse induction motor takes highstarting current if started at full voltage. In order tolimit this high starting current to reasonable limitsstarting methods are used.


3/18

Methods of Starting There are primarily two methods of starting the

induction motor:-a) Full voltage starting.

b) Reduced voltage starting.

Full voltage starting methods consist of:-

a) DOL (Direct-on-line starting)

Reduced voltage starting consist of:-

a) Stator resistor (or reactor) starting.

b) Auto-transformer starting.c) Star-delta starting.


4/18

DOL(Direct-on-line)starting

This method involves direct switching of

poly-phase stator on to the supply mains.

The motor takes starting current of 5 to 7 times itsfull load current depending upon its size anddesign.

Such large current of short duration dont harm therugged squirrel cage motor, but the high currentsmay cause objectionable voltage drop in power

supply feeding the induction motor These large voltage drop causes undesirable dip in

the supply line voltage, consequently affecting theother equipments connected to the same supply.


5/18

The relation between the starting torque Ts and fullload torque Tf is now obtained .

Let Is and If be the per phase stator currents drawn

from the supply mains corresponding to startingand full load conditions respectively.

We know:-

Therefore:-

------Eqn(1)

Now

s

rIT

s

e22

2..

1

f

f

s

ff

s

f

s sI

I

srI

rI

T

T.

1

2

2

2

2

2

sc

sc

st IZ

VI 1


6/18

V1 is per-phase stator voltage & Zsc=(r1+r2)+j(x1+x2), is the leakage impedance.

Therefore Eqn(1) can be written as:-

----Eqn(2)f

f

sc

f

s sI

I

T

T.

2


7/18

Stator resistor(reactor)method In this method, a resistor or a reactor is inserted in

between motor terminals and supply mains. At the time of starting some voltage drop occurs

across the starting resistor and therefore only afraction x of supply voltage appears across it.

This reduces the per phase starting currents Isdrawn by the motor from the supply mains.

As the motor speeds up, the

reactor is cut out in stepsand finally short-circuited

when the motor speed is

near to synchronous speed.


8/18

Since the per phase voltage is reduced to xV1 theper phase starting current is:-

Now we know:-

Therefore we have:-

-----Eqn(1)

Therefore:-

scsc

s

xIZ

xVI 1

s

rIT

s

2

2

2.1

f

f

sc

f

s sI

Ix

T

T2

2

2

2

1

1

switchingdirectwithtorquestarting

startingreactorwithtorquestartingx

V

xV


9/18

Auto-Transformer starting A fraction xV1 of the supply voltage V1 is applied to

the stator terminals at the time of starting, bymeans of an auto-transformer.

This reduces the motor current and also thecurrent drawn from the supply.

After the motor has accelerated near to its operatingspeed, auto-transformer

is disconnected and full

line voltage is applied by

connecting it directly

across the supply mains.

x is less then one.


10/18

With auto-transformer, per phase starting currentin motor winding is:-

-------Eqn(a)

If no load current is neglected , then per phaseoutput VA must be equal to its per phase input VA.That is:-

Is.V1 = xV1 (per phase starting current)\

Or Is. V1 = x V1 (x Isc)

Hence per phase starting current from the supply

mains,-------Eqn (b)

scsc xIZ

xV 1

scsIxI .

2


11/18

Now from the torque equation we have:-

---------Eqn(c)

Form eqn(b) we can write:-

-------Eqn(d)

f

f

sc

f

s sI

xI

T

T2

2)(

f

f

sc sI

Ix

2

2

f

f

s

f

s s

I

I

x

x

T

T2

2

2 .1

f

f

s sI

I

x

2

2

1


12/18

For an induction motor

Therefore:-

------Eqn(e)

2(Voltage)Torque

2

2

1

1

switchingdirectwithTorque

rtransforme-AutowithTorquex

V

xV


13/18

Star-Delta Starting This method is used in the case of motors which are

built to run normally with delta connected statorwindings.

It consist of a two way switch called TPDT

(Triple pole double throw)which connects the motorin star for starting and then in delta for normalrunning.

When star connected, the applied voltage over eachmotor phase is reduced by a factor of and hencethe torque developed becomes 1/3 of that whichwould have been developed if motor were directlyconnected in delta.

31


14/18

During starting period when motor is starconnected, it takes 1/3 as much starting current anddevelops 1/3rd as much torque as would have been

developed when it directly connected in delta.


15/18

Let VL be the line voltage. Then per phase motorstarting current Isy with stator connected in star isgiven by:-

With stator winding in star, Isy is also the startingcurrent with star -delta starter. If the stator windingis connected in delta then with direct switching, per

phase motor starting current Isd would be given by:-

And starting line current with direct switchingwould be equal to:

sc

L

syZ

V

I 3

scd

sc

L

sdI

Z

VI

scdsd

sc

L IIZ

V333


16/18

Note that :

Here the subscript y and d denotes star and delta

connections of the stator windings.

Therefore

Thus from the star-delta starter, the startingcurrent from the mains is one-third of that withdirect switching in delta

sdsy II3

1

31

3

3

1

3deltainswitchingdirectthcurrent wilineStartingstarterdelta-starthcurrent wilineStarting

sc

L

sc

L

sd

sy

Z

VZ

V

II


17/18

Also:

This shows that star-delta starter reduces thestarting torque to one-third of that produced by

direct switching in delta. Now ratio of starting and full load torque is given

by:-

3

13

deltainswitchingdirectwithtorqueStarting

starterdelta-starwithtorqueStarting2

2

L

L

V

V

2

2

f

s

Tdelta,indingstator winwithtorqueloadFull

Tstarter,delta-starwithtorqueStarting

fd

flsy

I

sI

fl

dfl

dscfl

fd

sd

s

I

Is

I

I 2

.

.

2

2

3

13

1


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THANKS!!

Starting Methods of Induction Machine

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